The invention relates to a tubular collector module with a header tube, with has an inlet and an outlet for the heat transfer medium, and having at least one collector tube, through which the flow is coaxial, and which has an envelope tube, an absorber tube, and a coaxial tube.
The collector tubes through which the flow is coaxial belong to the category of collector tubes with an absorber through which the flow is direct. These collector tubes have an envelope tube, an absorber tube, and a coaxial tube. These three tubes are disposed inside one another, with the envelope tube located outermost, the absorber tube located eccentrically or coaxially to the envelope tube, and the coaxial tube located coaxially to the absorber tube. The envelope tube and the absorber tube are sealed off from one another. In the interstice between the envelope tube and the absorber tube, there is either a vacuum or an inert gas. This serves above all to insulate the absorber tube from the environment. The absorber tube is coated with an absorber in order to absorb the solar heat. To transport this heat onward, a heat transfer medium flows through the absorber. In collector tubes in which the flow is coaxial, the heat transfer medium is introduced through a coaxial tube. This is a tube open on both ends, which enters the absorber tube through one of its two ends. The other end of the absorber tube is closed. The end of the coaxial tube that does not protrude into the absorber tube communicates with the inlet of the header tube. The heat transfer medium flows through the inlet into the coaxial tube and from there through the absorber tube and back again to the outlet. From the outlet of the header tube, the heat transfer medium is carried to a heat exchanger, where it gives up the heat to the reservoir.
Various tubular collectors through which the flow is coaxial are available on the market. One widely distributed tubular collector has separate lines for the inlet and outlet of the header tube. Flexible corrugated tubes are used as the inlet and outlet. The absorber tube and the coaxial tube are joined to the inlets and outlets via clamping ring screw fastenings. This type of assembly is very complicated and relatively expensive. The use of separate lines for the inlet and outlet also means increased space is required. Furthermore, if flexible corrugated tubes are used, a housing for the header system must also be provided.
The tubular collectors described above are as a rule constructed as modules comprising a plurality of collector tubes all connected to a common inlet and a common outlet. The individual tubular collector modules can be combined with one another via double O-ring plug-in connections.
In another tubular collector with a flow coaxially through it that is available on the market, the header tube also experiences a coaxial flow through it. Both the absorber tube and the coaxial tube, as well as the outer coaxial tube of the header tube, are embodied integrally as a whole. This does have the advantage that only the envelope tube has to be sealed off from the absorber tube. However, the production of these tubular collectors is very complicated.
A further tubular collector with a flow coaxially through it is described in German Utility Model DE 298 08 532 U1. The header tube in that reference is embodied as a header box assembly. The inlet and the outlet are described as two integrally embodied conduits extending parallel, which are separated only by a wall that has the applicable opening for the various coaxial tubes. The coaxial tube is inserted into the openings in the partition, and the absorber tube is received by an opening in the side wall of the header box, which wall communicates with the outlet conduit, and the envelope tube is received in an extension of the housing. All three tubes are joined by adhesive to the housing parts or header box parts. As a result, it is true that a more-compact design is achieved. However, the seal that is attainable by the adhesive has proved to be inadequate under a high thermal load.
German Utility Model 299 08 190 U1 describes a special clamping system for solar collectors. The vacuum tubes and the tubes of the heat transfer circulation system intersect, touching one another for heat transfer, at an angle of 90xc2x0; the intersecting tubes are joined to one another by positive engagement by means of a clamping system, comprising two jaws whose clamping jaws can be prestressed by a tensing mechanism.
German Utility Model DE 297 10 494 U1 discloses a solar collector with a heat pipe, which transfers the heat by evaporation and condensation of a heat-carrying liquid. To avoid overheating, a closure device of bimetal is provided, which if a threshold temperature is exceeded restricts the flow of the heat-carrying liquid from the condenser to the evaporator.
German Patent Disclosure DE 32 06 624 A1 involves a solar energy collector based on an air/water heat exchanger, in which a plurality of collector elements are connected together fluidically. To improve the air circulation, a blower is provided.
International Patent Disclosure WO 83/3891 discloses a tubular solar collector without coaxial tubes in the absorber tubes, which are inclined downward relative to a header tube. The absorber tubes are sealed off from the header tube by means of O-rings.
With this background, it is the object of the present invention to furnish a tubular collector module which intrinsically combines the advantages of simple assembly and good sealing.
This object is attained by a tubular collector module as defined by claim 1.
The tubular collector module of the invention can be assembled very simply. If the coaxial tube is not joined solidly to the absorber tube and to the envelope tube, then the coaxial tube is first introduced through the nipple into the receiving element of the inlet. Next, an elastic element is introduced into the absorber tube, into the end remote from the header system. The absorber tube is then inserted via the coaxial tube and via the nipple of the header system.
By means of the sealing element between the nipple and the absorber tube, no heat transfer medium can escape between the nipple and the absorber tube. The elastic element that is disposed in the absorber tube in turn presses the coaxial tube into the receiving element of the inlet. The heat transfer medium can now flow from the inlet into the coaxial tube, into the absorber tube on the opposite end of the coaxial tube, and back again between the nipple and the coaxial tube into the outlet.
If the collector tube is not joined to the absorber tube, then it is inverted over the absorber tube afterward; if the absorber tube and the collector tube are solidly joined to one another, then the absorber and the collector tube are simultaneously inverted over the coaxial tube and the nipple.
To enable assuring an unimpeded flow of the heat transfer medium even if the elastic element provides strong sealing, the elastic element is advantageously embodied as a spiral spring.
The sealing element is advantageously embodied between the nipple and the absorber tube as at least one O-ring. Thus not only is the best sealing achieved, but O-rings are also economical and simple to install. Moreover, by the selection of a suitable plastic, it is possible to optimize the service life, despite the thermal load during continuous operation.
Advantageously, the at least absorber tube and the at least one envelope tube are embodied integrally. This has the advantage that the vacuum or the inert gas atmosphere that prevails between the absorber tube and the envelope tube, for the sake of achieving good functioning of the solar collector, is not later impaired, for instance by leaks where the absorber tube is joined to the envelope tube.
The collector tube is the envelope tube that is permeable to sunlight. It comprises a transparent material. This is indeed generally a material of low thermal conductivity, but this property is not decisive, since the tube is intended not to heat up severely. Preferably, the envelope tube comprises glass. A low-iron glass (soda lime glass or borosilicate glass) with high solar transmission and low production costs, of the kind for instance used as a primary packaging means for pharmaceutical products, is preferred. The envelope tube can also comprise a transparent plastic, such as polymethylmethacrylate (PMMA). Glass has the advantages over plastic of higher diffusion resistance and higher UV stability; conversely, plastic compared to glass is less breakable and simpler to handle. In most embodiments, the envelope tube is provided with a reflector.
The absorber tube comprises a material with a high temperature resistance (up to at least 250xc2x0 C.) and low thermal conductivity. Materials with a low thermal conductivity are understood here to mean materials that have a specific thermal conductivity of xe2x89xa62 J/msK. Preferably, the internal tube is of glass, preferably borosilicate glass. It can also comprise a temperature-stable plastic. It is provided with an absorber.
Preferably, both the at least one absorber tube and the at least one envelope tube are of glass. The absorber tube and the envelope tube are advantageously fused eccentrically to one another. At the fusing point, to reduce stress in the glass, large radii are provided.
In a preferred embodiment, the header system is embodied as a header tube, which has at least one distributor plate which is disposed in the longitudinal direction in the header tube, so that the header tube is divided into at least one inlet chamber and at least one outlet chamber. The receiving element for the coaxial tube of the inlet chamber is disposed in the distributor plate. By embodying the header system as a single-tube system, only one joining point is needed for each connected collector tube. Moreover, the space required by a single header tube is less, and the installation is less complicated.
Preferably, the distributor plate is of metal. By means of the spring force of the elastic element, the metal distributor plate is additionally pressed against the coaxial tube. If there is a sudden pressure increase (for instance from steam hammer), the distributor plate is deflected by way of the contrary force of the compression spring. This creates a gap between the distributor plate and the header tube along the tube axis, as a result of which a pressure equilibrium between the inlet chamber and the outlet chamber is made possible.
The effect just described above is advantageously reinforced by a suitable fastening of the distributor plate in the header tube, for instance by providing that the distributor plate is joined to the header tube via only two opposed edges, preferably the edges of the distributor plate opposite one another in the axial direction of the header tube.
The distributor plate advantageously has at least one flap, which when it is open connects the at least one inlet chamber and the at least one outlet chamber, and when it is closed disconnects the at least one inlet chamber and the at least one outlet chamber. By means of such a flap on the distributor plate, which is not pressed against the header tube until as a result of the flow of the heat transfer medium, fast venting of the entire system is made possible. Before collection starts, the flap on the distributor plate enables convection between the coaxial tube and the absorber tube. This improves the heat transfer in the collector header tube.
In another preferred embodiment, the distributor plate is replaced by an internal tube disposed in the header tube. The internal tube forms the inlet chamber, and the volume between the internal tube and the header tube forms the outlet chamber.
On the one hand, the internal tube can be embodied cylindrically, and its rear end, in terms of the inlet flow direction, is closed. This closure can for instance be a disk or cap secured by soldering or some other type of fastening. On its open end, conversely, the internal tube is sealed off from the header tube by means of a sealing ring. For better stabilization, the internal tube can additionally be centered on its closed end relative to the header tube by means of a pierced ring. The openings in the centering ring serve to allow the heat transfer medium to flow out.
On the other hand, the internal tube can be embodied essentially cylindrically, and its rear end in terms of the inlet flow direction gradually converges, so that that end is closed. This can be accomplished for instance by pinching the internal tube. On its open end, an internal tube of this kind would be sealed off from the header tube by means of a conical fit.
It has proved to be especially advantageous for the at least one element for receiving the coaxial tube to be embodied either as a bore with a curved indentation or as an open bowl. Receiving elements embodied in this way serve both to guide the coaxial tube and to seal it off.
To enable venting the tubular collector module when the tubular collector module is put into operation, the internal tube advantageously has a small opening on its closed end.
Advantageously, the tubular collector module has a housing, in which both the header system and in part the at least one collector tube are accommodated. The housing serves to stabilize the tubular collector module mechanically as a whole. It is advantageous if the envelope tube is joined to the housing. The joining can be embodied by positive engagement by making a toroidal groove in the envelope tube in the housing, and either a toroidal bulge in the housing or an O-ring groove with O-rings is provided. Nonpositive connections are also possible, and they can for instance be embodied as an O-ring on the envelope tube and an O-ring groove in the wall of the housing.
Another possible way of joining the envelope tube to the housing is to attach clips, securing rings, and/or securing bars to the envelope tube that at the same time are let into the housing. For additional security, a hoop can be placed around the header system, particularly when it is embodied as an header tube, and this hoop can be secured to the safety bar. These securing elements absorb the forces that can occur for instance from the overpressure prevailing in the system or from the thermal changes in length of individual components.
In tubular collector modules with more than one collector tube, the individual collector tubes are preferably connected parallel to one another; all the collector tubes are supplied with heat transfer medium from the same inlet, and the heat transfer medium flows out of all the collector tubes into the same outlet. Individual tubular collector modules can be connected in series with one another.
The tubular collector modules can communicate with one another via a plug-in system attached directly to the header system. Double O-ring systems are especially preferred. It can also be advantageous to place a clamp around the connection.